In order for a path taught on one robot to be accurately replayed on a second robot of the same type, the second robot must be "mastered". What mastering does is adjust the joint angle counters of the robot so that when two different robots are in the same position relative to the world (i.e. the same configuration), the joint angle counters will be exactly equal. Thus, when the joint angles obtained from teaching one robot are given to another robot, the second robot will move to the same position as the first.
Mastering is also needed whenever any of the robot's drive train elements are serviced since this changes the relationship between the angle of the motor and the angle of the axis. Mastering resets the joint angle counters to account for this new relationship. Thus, paths taught with the old drive train can be played on the robot with the new drive train.
Another area where mastering is used is in off-line programming where the path of the robot is generated off-line and does not come from points taught on the robot. Mastering is used here to increase the absolute accuracy of the robot so that the robot's path closely follows the path generated off-line.
One way to master a robot is to identically align the output gears of the joints of each robot. This is done by placing a mastering hole in the output gears and pushing a pin through the gears and into a cover or casting. This method requires very accurate placement of the mastering holes in the gears and in the covers or castings. However, this method is still relatively inaccurate. This inaccuracy arises because a relatively small error at the small gear radius turns into a relatively large error at the radius at the end of the arm. Clearance between the pin and the hole is another source of error. Also, since the joint can be moved in either direction before the pin is put in, backlash errors cannot be eliminated. Another disadvantage of this method is that it does not place the robot end effector in a certain position, but rather, it merely places the output gears in a certain position. Any manufacturing errors in the arm itself, such as machining tolerances or extrusion warping turns into mastering errors, since they cannot be accounted for in this type of mastering scheme.
A more accurate way to master a robot is to adjust the joint angle counters so that they agree with the actual angles of the arm. The robot is placed in a certain configuration, then the joint angle counters are set to the angles corresponding to this configuration. This creates a certain reference position relative to the outside world.
Position of the robot during the mastering procedure must be very accurate and very repeatable, since this reference position is compared with all other positions of the robot. This method is much more accurate than the method using mastering holes and gears, since the arm itself is being placed in a repeatable position, not just the output gears. This eliminates some of the arm's manufacturing errors.
To ensure that the robot is in a very accurate and repeatable position, a mastering fixture is usually used. A mastering fixture is a very accurate structure that attaches to the robot base and has some means for placing the robot joints in an accurate position.
One type of mastering fixture uses six dial indicator gauges to measure the position of the robot wrist. A bar or other small structure is attached to the face plate of the robot. The robot operator then moves the robot so that the bar enters the mastering fixture and contacts the gauges. The robot is then moved so that all six gauges read some predetermined nominal value. When this occurs, the robot is in the "mastered position" and the joint angle counters are reset. This method is much more accurate than the scheme which uses holes in the gears but has many disadvantages.
One disadvantage of this method is that it is very slow. Since all six gauges must read their nominal value simultaneously, the robot must be slowly and carefully moved into the correct position. Another disadvantage is that the fixture is not very durable. If it is dropped or hit by the robot, the dial indicators may break or become maladjusted. The six dial indicators also greatly add to the cost of the mastering fixture.
One prior art mastering fixture is indicated in FIGURE and utilizes a series of pegs and V-notches to place a six axis robot into a mastered position. For example, two pegs 10 (only one of which is shown) are attached on opposite sides of a casting 12 of a wrist mechanism, generally indicated at 14. Two pegs 22 are attached to castings 24 and 26 of the wrist mechanism 14 for the last two joints of the wrist mechanism 14. A mastering fixture, generally indicated at 16, has two sets of precision V-notches 18 against which the pegs 10 simultaneously engage. The mastering fixture 16 also includes a pair of precision machined flats 20 which the pegs 22 engage.
An arm 28 of the robot 28 is moved either with a teach pendant or manually, so that the four pegs 10 and 22 contact all of the precision machined surfaces at the same time.
Clamps (not shown) are used to hold the pegs 10 in the V-notches 18 while the two other pegs 22 are moved into position. One of the problems with the mastering fixture 16 as illustrated in FIG. 1 is the amount of precision machining which adds to the cost of the fixture 16. The precision surfaces, peg holes and the pegs themselves must all be accurately machined.
The U.S. Pat. No. to Harjar et al 4,372,721 discloses a calibration fixture including, a stationary member having first, second and third mutually orthogonal flat locating surfaces; and a movable member having first, second and third flat mutually orthogonal locating surfaces configured to simultaneously contact respective first, second and third locating surfaces of the stationary member only when the movable member occupies a predetermined reference spatial position and orientation. Cam positioning means are jointly associated with the movable and stationary members to cam the respective first, second and third locating surfaces simultaneously into contact with each other to locate the movable member at the predetermined reference spatial position and orientation.
The U.S. Pat. No. to Harjar 4,552,502 discloses calibration by locking wrist links relative to each other. Such a calibration scheme does not account for differing link lengths.
The U.S. Pat. No. to Evans et al 4,362,977 discloses a calibration scheme utilizing a multitude of holes and datum surfaces, each one of which is used separately to obtain better absolute accuracy. The robot is moved under computer control to a nominal position and this position is measured using a calibration mask and then recording the errors for use at a later time. These errors are then compensated by software.
The U.S. Pat. No. to Szonyi 4,642,781 discloses a calibration system wherein commanded robot motion is compared to the actual measured motion and any differences are accounted for.
The U.S. Pat. No. to Jacobs et al 4,481,592 discloses the use of a calibration system including a calibration fixture which is attached to a robot base and wrist, for placing the wrist in a desired pose. The calibration fixture utilizes a single wrist attachment point. Actual link lengths are measured as part of the calibration procedure.